The following paragraphs contain some discussion, which is illuminated by the innovations disclosed in this application, and any discussion of actual or proposed or possible approaches in this Background section does not imply that those approaches are prior art.
Oil and gas hydrocarbons are naturally occurring in some subterranean formations. A subterranean formation containing oil or gas is sometimes referred to as a reservoir. A reservoir may be located under land or off shore. Reservoirs are typically located in the range of a few hundred feet (shallow reservoirs) to a few tens of thousands of feet (ultra-deep reservoirs). In order to produce oil or gas, a wellbore is drilled into a reservoir or adjacent to a reservoir.
A well can include, without limitation, an oil, gas, or water production well, or an injection well. As used herein, a “well” includes at least one wellbore. A wellbore can include vertical, inclined, and horizontal portions, and it can be straight, curved, or branched. As used herein, the term “wellbore” includes any cased, and any uncased, open-hole portion of the wellbore. A near-wellbore region is the subterranean material and rock of the subterranean formation surrounding the wellbore. As used herein, a “well” also includes the near-wellbore region. The near-wellbore region is generally considered to be the region within about 100 feet of the wellbore. As used herein, “into a well” means and includes into any portion of the well, including into the wellbore or into the near-wellbore region via the wellbore.
A portion of a wellbore may be an open hole or cased hole. In an open-hole wellbore portion, a tubing string may be placed into the wellbore. The tubing string allows fluids to be introduced into or flowed from a remote portion of the wellbore. In a cased-hole wellbore portion, a casing is placed into the wellbore, which can also contain a tubing string. A wellbore can contain an annulus. Examples of an annulus include, but are not limited to: the space between the wellbore and the outside of a tubing string in an open-hole wellbore; the space between the wellbore and the outside of a casing in a cased-hole wellbore; and the space between the inside of a casing and the outside of a tubing string in a cased-hole wellbore.
During well operations, it is common to introduce drilling fluids, sometimes referred to as drilling muds, into the wellbore. These drilling muds are used to provide hydrostatic pressure to prevent formation fluids from entering the wellbore, to keep the drill bit cool and clean during drilling, to remove drill cuttings, and to suspend drill cuttings when drilling is suspended when the drilling assembly is brought in and out of the wellbore. The specific drilling mud chosen is selected to avoid formation damage and to limit corrosion and can be selected from water-based muds, oil-based muds and gaseous muds.
During the drilling of subterranean wellbores, it is not uncommon to encounter strata that include reactive shales. As used herein, the term “shale” means materials such as certain types of clays (for example, bentonite) and related subterranean materials that may “swell,” or increase in volume, when exposed to water. Reactive shales may be problematic during drilling operations because of their tendency to degrade when exposed to aqueous media such as aqueous-based drilling muds. This degradation, of which swelling is one example, can result in undesirable drilling conditions and undesirable interference with the drilling mud. For instance, the degradation of the shale may interfere with attempts to maintain the integrity of drilled cuttings traveling up the well bore until such time as the cuttings can be removed by solids control equipment located at the surface. Degradation of drilled cuttings prior to their removal at the surface greatly prolongs drilling time, because shale particles traveling up the well bore break up into smaller and smaller particles, which increasingly exposes new surface area of the shale to the drilling mud, which leads to still further absorption of water, and further degradation.
Typical drilling muds generally include clays, heavy-weight additives and viscosifiers suspended in either a water or oil based fluid. The clays are generally a combination of native clays and bentonite, a three-layer clay that swells in the presence of water. Generally, when the drilling muds are being pumped into the wellbore, they are in a thin, free-flowing liquid phase. However, when the pumping is stopped, the static fluids form a gel that resists flow. It is generally very difficult to remove the drilling mud from the wellbore when it is in its gel form versus its liquid form.
During well completion, it is also common to introduce a cement composition into an annulus in a wellbore. For example, in a cased-hole wellbore, a cement composition can be placed into and allowed to set in an annulus between the wellbore and the casing in order to stabilize and secure the casing in the wellbore. By cementing the casing in the wellbore, fluids are prevented from flowing into the annulus. Consequently, oil or gas can be produced in a controlled manner by directing the flow of oil or gas through the casing and into the wellhead. Cement compositions can also be used in primary or secondary cementing operations, well-plugging, squeeze cementing, or gravel packing operations.
Generally, the presence of gelled drilling mud impairs the rheological and mechanical properties of the cement composition, such as compressive strength development and the ability of the cement composition to set properly in the wellbore. Such situations can lead to poor bonding between the cement composition and the formation, as well as between the cement composition and the casing. Typically, a spacer fluid is circulated in the wellbore to remove the drilling mud before cementing operations; however, such spacer fluids do not entirely remove the drilling mud, especially any gelled drilling mud. Therefore, a drilling mud that could remain in fluid form in the wellbore and mix with cement compositions without impairing the rheological, cementitious and mechanical properties of a cementing operation would be advantageous.